According to the "Glossary of Terms used in the Pressure Sensitive Tape Industry", a pressure sensitive adhesive (PSA) is a material which is aggressively and permanently tacky, adheres without the need of more than finger pressure, exerts a strong holding force, and has sufficient cohesiveness and elasticity that it can be removed from substrates without leaving a residue. Tackified acrylic copolymers are used as PSAs. Such tackified copolymers have usually been prepared by physically mixing a tackifying petroleum-based hydrocarbon resin or a natural resin with a polymerized acrylic copolymer. While the resulting adhesive is often satisfactory to obtain certain properties such as tackiness and peel strength on substrates such as polyester or stainless steel, certain properties are found to be deficient, particularly the holding power on these same substrates. Previously, increasing the peel strength of tackified acrylic copolymers to a value greater than 2.0 pounds per inch caused the holding power (1 kg over one inch square on stainless steel) to diminish to below 40 hours. Of course, it is desirable to maintain high holding power, particularly when the PSA is to be used to prepare an adhesive tape.
The prior art describes attempts to prepare satisfactory tackified acrylic copolymers. For example, Japanese Patent J-59213783 teaches the preparation of a hot-melt PSA by first heating a tackifying resin having a softening point between 60.degree. and 200.degree. C. to above its melting point and adding to the hot melt a polymerization mixture of alkyl (meth) acrylate, a functional monomer such as acrylic acid, and a free radical initiator. The polymerization mixture is added to the hot-melt over a period of several hours, with stirring, to form a pale yellow, transparent, solid, hot-melt adhesive. The tackifying resins are broadly identified to include rosin-based resins, terpene-phenol resins, phenol resins, coumarone resins, aliphatic and aromatic resins.
Japanese Patent J-59227967 discloses a hot-melt polymerization of an alkyl (meth) acrylate monomer and a functional comonomer including (meth) acrylic acid, maleic anhydride, maleic acid, vinyl ethers, and the like where a surfactant is present with the resin during the polymerization to form a solid mass, hot-melt adhesive.
In Japanese Patent 53074041 a polymerized powdery toner product is obtained by dissolving a binder resin such as a vinyl resin, acetal resin, epoxy resin, or the like in a polymerizable liquid monomer such as styrene, vinyl toluene, (meth) acrylic acid or its ester and the like and polymerizing the monomers in bulk in the presence of a coloring material.
In Japanese Patent J-51125472, a petroleum resin emulsion is obtained by polymerizing vinyl monomers in the presence of petroleum resins having softening points of from 40.degree. to 160.degree. C., an average molecular weight of 300 to 3000, and an acid value and saponification value of less than 1. The monomers include, for example, alkyl (meth) acrylates, vinyl acetates and vinyl chlorides, styrene, acrylonitrile, and acrylic acid. The emulsified mixture is then reacted in an emulsion polymerization reaction to form a shelf-stable emulsion adhesive. The resin emulsion produced is described as having fine particle sizes and ample stability and when cured, the films produced have excellent water resistance and gloss. Also, U.S. Pat. No. 4,645,711 to Winslow et al. describes the incorporation of hydrocarbon resins from many sources, i.e., from hydrogenated resin esters, polyterpene, polymerized alkyl styrene, and polymerized petroleum-derived monomer resins, into PSA tape compositions where the adhesive is a polymerized acrylic emulsion. The patent describes physically mixing the resin with the polymer emulsion.
Numerous approaches have been used to produce resin emulsions. One approach is to dissolve the resin in a hydrocarbon solvent, combine the resin solution and water to form an emulsion, and strip the solvent. Invariably some residual hydrocarbon solvent remains in the finished emulsion which is undesirable in certain applications. This has led to the development of solvent-free dispersions (see U.S. Pat. No. 2,809,948) and emulsions (see U.S. Pat. No. 3,377,298) of petroleum resins. In both of these formulations, ionic emulsifiers have been utilized; in the former a mixture of cationic and non-ionic surface active agents is used to achieve a resin emulsion; and in the latter an ionic surfactant is used in combination with an aqueous gel of a swelling earth to produce an emulsion paste of a petroleum resin. These resin emulsions have been commercially used to tackify natural rubber, carboxylated styrene-butadiene and acrylic latexes for many adhesive applications.
As mentioned above, there has generally been a decrease in shear properties associated with an increase of peel strength and tackiness. There is a need to improve the peel strength of PSAs while yet maintaining high shear properties and thus obviating the decline in shear usually resulting from increase in peel.
Accordingly, it is an object of this invention to prepare a resin tackified acrylic copolymer in the form of a latex-like dispersion for application as PSAs having increased peel strength without serious decrease in shear.
It is a further object to provide a process for preparing a resin tackified acrylic copolymer, wherein the tackifying resin is dissolved in the acrylic monomer solution prior to the polymerization reaction, and wherein the tackifying resin is essentially free of chain transfer agents such as allylic hydrogen and conventional antioxidants.
It is still a further object of this invention to provide PSAs for the manufacture of tapes and adhesives for the manufacture of laminant articles.